AVS 70 Session 2D+EM+MI+QS-WeM: 2D Materials: Heterostructures, Twistronics, and Proximity Effects

2D materials have attracted broad interest due to novel properties that arise in atomically thin crystals. As interesting scientifically and important technologically, but much less explored are van der Waals (vdW) crystals that, assembled from 2D building blocks, lie between a monolayer and the bulk. In this regime, phenomena such as phase separation, transformations between crystal polymorphs, and competition between different stacking registries provide unprecedented opportunities for controlling morphology, interface formation, and novel degrees of freedom such as interlayer twist. But going beyond a single layer also poses significant challenges, both due to the diversity of the possible few-layer structures and the difficulty of probing functionality such as optoelectronics and ferroics at the relevant length scales.

Here, we discuss our recent research that addresses these challenges focusing on group IVA chalcogenides, an emerging class of anisotropic layered semiconductors promising for energy conversion, optoelectronics, and information processing. Advanced in-situ microscopy provides insights into the growth process, interlayer twisting,and emerging functionality such as stacking-controlled ferroelectricity. Nanometer-scale electron excited spectroscopy identifies photonic light-matter hybrid states and reveals anisotropic and valley-selective charge carrier flows across interfaces in heterostructures. Our results highlight the rich sets of materials architectures and functionalities that can be realized in van der Waals crystals and heterostructures beyond the 2D limit.

Over the last few years, significant efforts have been made in exploring low-dimensionality materials such as single layer Graphene (SLG), transition metal dichalcogenides (TMDCs) and carbon nanotubes (CNTs), for a wide range of applications covering beyond CMOS logic, EUV pellicles, photonics, and sensing (amongst others). Due to their intrinsic 2D or 1D nature, these materials are highly sensitive to processing damage leading to stoichiometric changes or crystalline defects. Among the many manufacturing steps required for building devices, the cleaning of these systems is an absolute requirement and a bottleneck. Typically, during processing, residual polymers or carbon of ambient origin, do contaminate the surface leading to nanometric deposits that change the intrinsic transport/optical properties of the materials, and cause parasitic dielectric drift or high contact resistance. Wet cleaning, using organic solvents, is a mainstream approach, which proves to be inefficient for irreversibly physically adsorbed polymer residues. In this paper, we explore dry cleaning approaches, based on plasma treatment and UV cure. Plasma-based cleaning proves to be very efficient but leads to material damage, which can be minimized by tuning the average ion energy, the processing temperature, the plasma chemistry or adding a post-cleaning restoration step. For TMDCs, damage consist essentially in the creation of chalcogen vacancies, which lead to metal oxidation upon ambient exposure. For Graphene and CNTs, damage consist in carbon vacancies, causing lattice distortions, oxidation and ultimately a dramatic change of the material’s transport properties. Part of this presentation will explore the use of UV cure, which is a known method for cleaning polymers from semiconductor surfaces.

Quantum computers (QC) are poised to revolutionise computational capabilities by naturally encoding complex quantum computations, thereby significantly improving computation time compared to silicon-based technologies. Currently, Q-bits, the essential components of QCs, are made from conventional superconductors that operate efficiently only near absolute zero temperatures. To address this limitation, two-dimensional van der Waals (vdW) encapsulated high-temperature superconductor (HTSC) stacks have been proposed as future Q-bit candidates, driven by recent advancements in nanofabrication techniques. However, a detailed understanding of their structural and electronic properties is crucial.

We present low-temperature resonant soft X-ray investigations on ultrathin Bi₂Sr₂CaCu₂O_8+y (BSCCO) vdW heterostructures, promising candidates for large-scale Q-bit applications. BSCCO crystals exhibit two incommensurate lattice modulations (ILMs), providing an excellent opportunity to explore the relationship between structure and electronic behaviour in low dimensions. We report ILMs (Cu L₃) and structural peak (off resonance) maps obtained across the superconducting transition temperature (T_c ≈ 60 K). These signals, under external gating, present significant potential for further exploration offering new insights into the electronic interactions in vdW HTSC systems.

Transition metal dichalcogenides (TMD) are three-atom-thick layer materials that possess a wide array of properties, such as insulating, semiconducting, conducting, and superconducting. While there has been a significant amount of research on TMD for various applications including energy storage, photovoltaics, biomedicals, catalysis, hydrogen production processes, and healthcare, the usage of TMD for electronic applications has been researched most in the past two decades.

Each layer of TMD is a 2D sheet with the thickness of ~6-7Å. These layers are bonded by Van der Waals force. The ultra-thin structure enables TMD for semiconductor industry applications, which continuously requires both size-scaling of materials layers and electrical performance improvement of devices. The semiconductor-range band gap and the high electron/hole mobility of several TMD, such as MoS₂, WS₂, WSe₂, and MoSe₂allow their usage for high mobility ultra-thin channel transistor. In addition, the relatively high conductivity of some other TMD, such as TaS₂ and NbS₂ make them promising candidates for interconnect barrier/liner as a replacement of TaN/Ta bilayer.

TMD materials need to pass certain quality requirements to provide desirable property/performance; therefore, method of synthesizing TMD plays an important role for high quality materials. Solution-based deposition, non-vacuum electrodeposition, polymer-assisted deposition, physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD) have been used to deposit ultra-thin TMD. Among these techniques, CVD is the most popular deposition method, and to date, the highest quality of TMD for semiconductor applications is CVD-TMD. However, the need of lower thermal budget, better layer controllability, uniformity, and conformality requires the semiconductor community to explore ALD methods. In this presentation, we will review multiple ALD processes for TMD including precursors, process requirements for high mobility channel and barrier/liner applications. We will highlight challenges of TMD applications for logic, memory, and interconnects. The presentation will also feature our recent work on ALD MoS₂ for high-mobility channel transistor with >5 decades On/Off ratio and >1 µA/µm Ion. Our achievement of 300mm ALD MoS₂ deposition brings ultra-thin TMD materials closer to a manufacturable fab process for semiconductor industry.

Fe_5-xGeTe₂ is a centrosymmetric, layered van der Waals (vdW) ferromagnet that displays Curie temperatures T_c (270-330 K) that are within the useful range for spintronic applications. Little is known about the interplay between its topological spin textures (e.g., merons, skyrmions) with technologically relevant transport properties such as the topological Hall effect (THE), or topological thermal transport. We found via high-resolution Lorentz transmission electron microscopy that merons and anti-meron pairs coexist with Néel skyrmions in Fe_5-xGeTe₂ over a wide range of temperatures and probe their effects on thermal and electrical transport [1]. It turns out that we detect a THE, even at room T, that senses merons at higher T’s as well as their coexistence with skyrmions as T is lowered, indicating an on-demand thermally driven formation of either type of spin texture.Remarkably, we also observe an unconventional THE, i.e., in absence of Lorentz force, and attribute it to the interaction between charge carriers and magnetic field-induced chiral spin textures. We find that both the anomalous Hall effect (AHE) and THE can be amplified considerably by just adjusting the thickness of exfoliated Fe_5-xGeTe₂, with the THE becoming observable even under zero magnetic field due to a field-induced unbalance in topological charges [2]. Using a complementary suite of techniques, including electronic transport, Lorentz transmission electron microscopy, and micromagnetic simulations, we reveal the emergence of substantial coercive fields upon exfoliation, which are absent in the bulk, implying thickness-dependent magnetic interactions that affect the topological spin textures (TSTs). We detected a ‘magic’ thickness of t ~30 nm where the formation of TSTs is maximized, inducing large magnitudes for the topological charge density, and the concomitant AHE and THE resistivities at T ~ 120 K. Their values are observed to be higher than those found in magnetic topological insulators and, so far, the largest reported for 2D magnets. The hitherto unobserved THE under zero magnetic field could provide a platform for the writing and electrical detection of TSTs aiming at energy-efficient devices based on vdW ferromagnets.

[1] B. W. Casas et al., Adv. Mater. 35, 202212087 (2023).

[2] A. Moon et al., ACS Nano 18, 4216-4228 (2024).